1. Technical Field
The technology described herein is generally related to the field of semiconductor devices and, more particularly, to seal rings for integrated circuit devices having core circuitry generating both analog and digital signals.
2. Description of Related Art
Integrated circuit (IC) devices, each commonly having an area of less than one square inch, are fabricated simultaneously in multiples on a larger wafer, commonly having a cross-dimension of many inches. The individual IC devices, also known as “chips” or “die,” are segregated by “scribe lines” regions where the wafer can be sawed into individual chips once the fabrication is complete. Conventionally, surfaces of the chips are generally protected by the deposition of an uppermost passivation layer. Also conventionally, “seal rings,” also known as “guard rings,” generally are formed of at least one metal band around at least the upper the periphery of each chip as part of the fabrication of the IC dice prior to sawing the wafer. These seal rings separate each die from surrounding scribe lines, providing structural reinforcement and stopping undesirable moisture and mobile ionic contaminants from the scribe regions (also known as “auto-doping”) from entering chip active circuitry regions and affecting operational viability.
Moreover, conventionally it is believed that these seal rings should be electrically tied to the lowest (most negative) chip potential, or grounded. One reason is to prevent corrosion of die interconnects should the seal ring and any area of the core circuitry become in effect a galvanic cell due to the presence of moisture. In other words, if a seal ring were tied to a higher potential, there is the possibility that if there are cracks or pinholes in the chip passivation layer over interconnect metal of a lower potential, then metal at a crack or pinhole would become a galvanic cell anode of very small area compared to the relatively large area of the ring—acting as a galvanic cell cathode—and rapid corrosion at the crack or pinhole could occur.
In modern uses, chips may combine core circuitry—analog circuits, digital circuits, and power circuits—on a single die. In these mixed analog-digital and analog-digital-power IC devices, since the seal ring conventionally completely encircles the chip core circuitry, one problem is that digital circuitry electrical noise can affect operation of analog circuitry. FIG. 1 (Prior Art) is a simplified, schematic, block diagram illustrating such a die 80 (FIG. 1 is also in common assignee's U.S. Pat. No. 6,395,591 (McCormack et al.) as FIG. 4). Generally, each of said die core circuits would have input/output (I/O) connection circuitry, an I/O pad 84 for active signals, and a ground connection (or, alternatively to electrical ground, a connection to the chip's lowest negative electrical potential). Seal rings generally are connected electrically to a wafer substrate which supports the backside of each chip; for example, complementary metal oxide semiconductor (CMOS) devices have a P-type doped substrate connected to the most negative potential for the circuitry, usually a ground potential. This short-circuiting of the seal ring may provide additional electrostatic discharge protection for the chip core circuitry. In FIG. 2 (Prior Art), a schematic, highly-enlarged elevation view of a small-segment 200 of a mixed analog-digital signal IC die is illustrated. The chip is shown as having a known manner metal oxide silicon field effect transistor (MOSFET) 201, part of the digital circuitry, including “Digital GND (S/B)” taps 202 and a “Seal Ring” 203 which also is acting as “Analog GND.” The Seal Ring 203 separates the active components of the core circuits from the “Scribe Region” 205. The Seal Ring 203 is shown with a low resistive contact to a “P+ type Substrate” 204 through “P+,” “P-Well” and isolation “ISO P” layers. In other words, in accordance with the conventional wisdom, the metal seal ring effectively is shorted to ground through a low resistance path as shown in FIG. 2 (Prior Art)—namely, via the subjacent “P+” implant region to the “P-Well” implant to the “ISO P” buried layer to the “P Substrate.” Thus, the connection between the seal ring and substrate may be said to be “ohmic,” having a low resistance path to ground, i.e., short-circuited.
It is known that digital circuits and power circuits are in general electrically noisy. On the other hand, analog circuits are generally both quiet and sensitive to electrical noise. In mixed signal integrated circuits such as illustrated by FIG. 1, noisy power and digital core circuit blocks adjacent to the seal ring can couple noise into the quiet analog circuit block via the seal rings ohmic contact to the substrate, disrupting analog operations.
The problem is exacerbated in combined bipolar-CMOS (BiCMOS) chips where a richly doped buried isolation layer exists which forms an excellent conduction path between the substrate and the seal ring(s).
McCormack et al. provide a SELECTIVE SUBSTRATE IMPLANT PROCESS FOR DECOUPLING ANALOG AND DIGITAL GROUNDS, providing a decoupling of power supply noise, such as ground noise, between noisy and noise sensitive circuits within the chip core while also providing immunity against latch-up and electrostatic discharge.
Considering the ever-present design goal of shrinking die size, there is a need for improving the isolation between electrically noisy and noise sensitive semiconductor core circuits.
The present invention addresses such problems.
Many publications describe the details of commonly known techniques used in the fabrication of integrated circuits that can be generally employed in the fabrication of complex, three-dimensional, IC structures; see e.g., Silicon Processes, Vol. 1-3, copyright 1995, Lattice Press, Lattice Semiconductor Corporation, Hillsboro, Oreg.; Ghandhi, S. K., VLSI Fabrication Principles, copyright 1983, John Wiley & Sons, or Semiconductor & Integrated Circuit Fabrication Techniques, Reston Publishing Co., Inc., copyright 1979 by the Fairchild Corporation. Moreover, the individual steps of such a process can be performed using commercially available IC fabrication machines. The use of such machines and common fabrication step techniques may be employed in accordance with practicing the present invention and will be referred to herein as simply, for example, “ . . . in a known manner . . . .” As specifically helpful to an understanding of the present invention, approximate technical data are disclosed herein based upon current technology; future developments in this art may call for appropriate adjustments as would be apparent to one skilled in the art. Those techniques can be generally employed in the fabrication of the structure of the present invention. Moreover, the individual steps of such a process can be performed using commercially available integrated circuit fabrication machines. As specifically helpful to an understanding of the present invention, approximate technical data are set forth based upon current technology. Future developments in this art may call for appropriate adjustments as would be obvious to one skilled in the art.